1. Field of the Invention
This invention relates to an apparatus and method for generating essentially uniform compression in a relatively high-viscosity liquid, such as molten polymer, by squeezing the high-viscosity liquid between two parallel, porous plates which are each coated with a thin film of a relatively low-viscosity liquid having essentially uniform thickness and pressure, which are preferably maintained by continuously passing the low-viscosity liquid through the porous plate to a downstream face of the porous plate, at or near which the thin film is formed.
2. Description of Prior Art
Equibiaxial extensional (EBEX) flow relates to deformation of a material element by stretching the material equally in two directions while compressing the material in the third direction. Many polymer processing technologies, such as blow molding, compression molding and foamed polymer production, require simple and reliable methods for obtaining rheological flow data, which are not currently available.
Compression of molten polymers between unlubricated surfaces is conventional technology within the polymer processing industry and is often referred to as compression molding. In a compression molding process, a charge of molten polymer is positioned between two opposing solid surfaces that usually accommodate heat transfer from the molten polymer. A position of one of the two solid surfaces is normally fixed while a linear drive motor controls a variable position of the other solid surface. During the compression molding process, the distance between the two solid surfaces decreases over time.
In a compression molding process, the fixed solid surface is often mounted to a force transducer, so that it is possible to measure the force required to squeeze or compress the molten polymer. Measurement equipment, such as a displacement transducer, is often used to measure a distance between the opposing solid surfaces, under a given load. Compression molding is used to convert a molten polymer charge to a solid piece intermediate or final product that has a desired shape and thickness.
Compression molding technology produces nonhomogeneous deformation of the polymer material. Adhesion of the polymer to the solid surfaces results in shear deformation near the solid surfaces, thereby causing nonhomogeneous deformation, and nearly shear-free, or extensional, deformation in a central plane of the polymer material. Material near a central portion of the charge experiences relatively small deformations while material near an outer radial edge of the charge experiences relatively large deformations. Non-uniform deformation often produces different degrees of molecular orientation in compression molded pieces and thus non-uniform physical properties, as well as residual stresses that can diminish mechanical performance.
Unlubricated compression molding has been used to measure Theological properties of high-viscosity liquids, such as in connection with a conventional device referred to as a Williams Parallel-Plate Plastometer, which forms the basis for ASTM D926. The force or displacement resulting from a given displacement or force, respectively, can be measured and used to indicate rheological behavior of the polymer material. For Newtonian fluids, shear viscosity of the test material is simply related to the measured force and distance between the solid surfaces. However, for non-Newtonian fluids, such as molten polymers, there are no relatively simple relationships between measurable quantities, such as force and distance, and rheological properties. Thus, unlubricated compression technology is a relatively ineffective method for rheological analysis of relatively complex fluids.
Lubricated squeezing flow (LSF) methods, conventional technology, involve squeezing a viscous liquid between solid surfaces coated with a relatively thin film of a significantly lower viscosity liquid, such as a lubricant. The LSF method was developed for the purpose of generating equibiaxial extension in molten polymers. Lubricant films allow the molten polymer to slide past solid surfaces and thereby minimize shear flow in the melt, and thereby attempt to achieve uniform compression of the molten polymer. A lubricant film with a specified thickness is positioned between the molten polymer charge and the solid surfaces. The lubricant is usually a silicone oil which has a viscosity 1,000 to 100,000 times lower than the shear viscosity of the material, such as molten polymer melt.
In the known LSF method, as the material is compressed, lubricant flow from the film is driven by two types of flow: drag flow resulting from the flow of the polymer melt, and pressure-driven flow which is induced by the decreasing lubricant thickness. During compression of the melt, the lubricant film loses its effectiveness to prevent shear flow from occurring in the melt. The LSF method produces a relatively large pressure gradient over the lubricant film, causing the pressure in the lubricant to increase so that the force is no longer a simple function of the equibiaxial stress difference. The conventional LSF method is limited to relatively small deformations, such as where the equibiaxial Hencky strain is approximately 0.5. Thus, the LSF method is not particularly useful for the study of non-linear rheological behavior.
Other conventional methods have attempted to replace the lubricant loss over time in the LSF method, by injecting additional lubricant to the film through a single hole in a center of the plate. However, lubricant injection through a single hole in the center of the plate creates a substantial pressure gradient in the lubricant film and thus is not a viable solution to the lubricant thinning problem associated with conventional LSF technology.
The lubricant film between the two solid surfaces enhances but does not achieve uniformity of the pressure throughout the polymer material. However, there is an apparent need for an improvement to the existing LSF technology, the result of which generates a generally uniform pressure throughout the film positioned between the molten polymer charge and the solid surface.